This invention relates to the preparation of concentrated solutions of acrylonitrile polymer in organic solvents. More particularly, it relates to the rapid preparation of concentrated, clear, bubble-free, acrylonitrile polymer solutions of satisfactory color.
For many years spinning solutions for acrylic fibers have been prepared commercially by a process in which a slurry of acrylonitrile polymer in cold dimethylformamide or other suitable solvent is formed, held in a tank at about 70.degree. C. and then heated to about 150.degree. C., a process requiring several hours for formation of a clear solution. In practice, this process is limited to the preparation of solutions having a concentration of up to about 30% or slightly higher; attempts to reach higher levels of solids result in incomplete solution.
In Japanese Appln. Publn. No. 48-222328/1973 a method for forming acrylic solutions is disclosed in which a twin-screw processor is employed having a feeding section with a transport screw, a stirring section for forming a cold slurry of the acrylic polymer in the solvent, and a heating-dissolving section containing another transport screw in which the slurry is heated to 70.degree.-100.degree.C. to dissolve the polymer in the solvent. By the use of this method relatively rapid formation of solution is achieved; however, the concentration of solution of good quality which can be prepared is limited to about 30%, as in the method described above. Dieter Gras, in Plastics Design and Processing, August, 1972, pages 24-29, discloses a variation of this process in which the slurry is mechanically worked by kneading elements in the twin-screw processor while being subjected to a temperature substantially higher than the boiling point of the solvent. However, the use of this process leads to solutions having a color higher than is desirable.
A practical method for producing high quality concentrated solutions of acrylonitrile polymer in organic solvents has been greatly desired. Three parts of solvent are required for each part of polymer to make a 25% solution, while only two parts of solvent are required for a 33 1/3% solution and only one part for a 50% solution.
In accordance with the present invention it has been found that clear, bubble-free acrylonitrile polymer solutions of satisfactory color, having a solids concentration by weight of up to 50% or even 75%, can be produced at high speeds. The solutions are prepared by a novel process in which a cold slurry of acrylonitrile polymer in an organic solvent is prepared in the feed end of a twin-screw processor and is then heated to a critical temperature range in the discharge end of the processor while it is intermittently squeezed in volume in a direction transverse to the axes of the screws and thereby caused to sharply accelerate in their axial direction.